Three ruling party lawmakers have indicated that they could challenge front-runner Taro Aso to replace Japanese Prime Minister Yasuo Fukuda.
Kaoru Yosano, the economics minister, and Yuriko Koike, a former defence chief, have suggested they could run.
Nobuteru Ishihara, the son of Tokyo's controversial right-wing governor, has also indicated an interest.
Mr Fukuda stepped down on Monday, in an unexpected move widely criticised by the Japanese press.
He had been facing rock-bottom approval ratings, something a cabinet reshuffle and an economic stimulus package failed to improve.
The opposition has urged the government to call a snap election, but so far it has ruled that out.
Instead the party will elect its new leader - who automatically becomes the prime minister - on 22 September.
Governor's son
Taro Aso, 67, the hawkish secretary-general of the ruling Liberal Democratic Party, is seen as the front-runner.
But three more lawmakers look poised to join the race.
Seventy-year-old Kaoru Yosano, the economics minister who advocates higher taxes in a bid to reduce debt, says he wants to run.
Yuriko Koike, who was Japan's first female defence minister, says she is being encouraged to enter her name.
The third possible candidate is a former transport minister, 51-year-old Nobuteru Ishihara.
He is the son of Tokyo Governor Shintaro Ishihara, who is known for his nationalist views and frequent controversial statements.
Formal campaigning for the leadership election begins on 10 September.
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